This invention relates to methods and apparatus for the production of fused silica optical members. More particularly, the invention relates to methods and furnaces for the production of high purity fused silica having high internal transmission.
As practiced commercially, fused silica optical members such as lenses, prisms, photomasks and windows, are typically manufactured from bulk pieces of fused silica made in large production furnaces. In overview, silicon-containing gas molecules are reacted in a flame to form silica soot particles. The soot particles are deposited on the hot surface of a rotating or oscillating body where they consolidate to the glassy solid state. In the art, glass making procedures of this type are known as vapor phase hydrolysis/oxidation processes, or simply as flame deposition processes. The bulk fused silica body formed by the deposition of fused silica particles is often referred to as a xe2x80x9cboule,xe2x80x9d and this terminology is used herein with the understanding that the term xe2x80x9cboulexe2x80x9d includes any silica-containing body formed by a flame hydrolysis process.
FIG. 1 shows a prior art furnace 100 for producing fused silica glass. The furnace includes a crown 12 and a plurality of burners 14 projecting from the crown. As noted above, silica particles are generated in a flame when a silicon containing raw material together with a natural gas are passed through the plurality of burners 14 into the furnace chamber 26. These particles are deposited on a hot collection surface of a rotating body where they consolidate to the solid, glass state. The rotating body is in the form of a refractory cup or containment vessel 15 having lateral walls 17 and a collection surface 21 which surround the boule 19 and provide insulation to the glass as it builds up. The refractory insulation ensures that the cup interior and the crown are kept at high temperatures.
The prior art standard furnace further includes a ring wall 50 which supports the crown 12. The furnace further includes a rotatable base 18 Mounted on an oscillation table 20. The base is rotatable about an axis 3. The crown 12, the ring wall 3, the base 18 and the lateral walls are all made from suitable refractory materials.
The cup or containment vessel 15 is formed on the base 18 by means of lateral cup walls or containment walls 17 mounted on the base 18, which forms the cup or containment vessel 15. The lateral cup or containment walls 17 and the portion of the base 18 surrounded by the walls 17 is covered with high purity bait sand 24 which provides collection surface 21 for collecting the initial silica particles produced by the burners 14. During deposition and consolidation of the silica particles into a solid glass, the boule 19 is formed having sidewalls 23 and an upper major surface 25. As the boule 19 is formed during the deposition process, the upper major surface 25 of the boule 19 becomes the collection surface 21a for the silica particles, and as the thickness of the boule 19 increases during the deposition process, the distance z between the burners 24 and the collection surface decreases. The lateral walls 17 can be made from refractory blocks such as alumina base block for forming the walls 17 and an inner liner made of a suitable refractory material such as zircon or zirconia.
Surrounding the lateral walls 17 of the cup or containment vessel 15 is a shadow wall or air inflow wall 30. The shadow wall 30 is mounted on x-y oscillation table 20 by feet 40, for example four feet equally spaced around the circumference of the shadow or air inflow wall 30. Details on the construction a shadow wall and a furnace using a shadow wall may be found in U.S. Pat. No. 5,951,730, the entire contents of which are incorporated herein by reference. Other ways of mounting the air inflow wall to the oscillation table can be used if desired. The stationary ring wall 50 surrounds the ring wall and supports the crown 12. A seal 55 is provided between the stationary ring wall 50 and the air flow wall or shadow wall 30. The seal 55 includes an annular plate 56, which rides in or slides in an annular channel 58 formed within the stationary ring wall 50. The annular channel 58 can include a C-shaped annular metal plate which forms the bottom of the stationary wall, other forms of motion-accommodating seals can be used if desired, including flexible seals composed of flexible metal or refractory cloth, which, for example, can be in the form of bellows.
The products of combustion in a standard prior art furnace 100 are exhausted through ports 60 circumferentially spaced around the furnace. In a typical furnace, six ports 60 are provided, and the ports 60 are located between crown 12 and the top edge 50a of the stationary wall, such that the ports 60 are located above the deposition surface 21 and 21a during formation of the boule.
Boules typically having diameters on the order of five feet (1.5 meters) and thicknesses on the order of 5-10 inches (13-25 cm) can be routinely produced in large production furnaces of the type shown in FIG. 1. Multiple blanks are cut from such boules and used to make the various optical members referred to above. The blanks are generally cut in a direction parallel to the axis of rotation of the boule in the production furnace, and the optical axis of a lens element made from such a blank will also generally be parallel to the boule""s axis of rotation in the furnace. For ease of reference, this direction will be referred to as the xe2x80x9caxis 1xe2x80x9d or xe2x80x9cuse axisxe2x80x9d.
As the energy and power output of lasers increase, the optical members such as lenses, prisms, photomasks and windows, which are used in conjunction with such lasers, are exposed to increased irradiation levels and energies. Fused silica members have become widely used as the manufacturing material for optics in such high energy laser systems due to their excellent optical properties and resistance to damage at higher power levels.
The next generation of fused silica glass used in the microlithography market will require ArF (193 nm) internal transmission exceeding 99.65%/cm, and preferably exceeding 99.75%/cm. The standard manufacturing processes described above is capable of consistently producing fused silica lens blanks with 99.5%/cm. Reduction of metal contaminants, which have a major impact on UV transmission, has played a major role in the production of higher transmission fused silica. The effects of metals, such as sodium, potassium and iron, are evident at the 10""s of parts per billion level. The standard process has demonstrated the ability to produce fused silica having transmission of 99.65%/cm, without sacrificing glass homogeneity, but not in the quantity needed to make large production quantities of lens blanks and not with the consistency to serve as a basis for a production process. Accordingly, it would be desirable to provide methods and apparatus capable of consistently manufacturing large production quantities of fused silica having internal transmission equal to or greater than 99.65%/cm at 193 nm, and preferably greater than 99.75%/cm.
The invention relates to methods and apparatus for producing fused silica. According to one aspect of the invention, a method for producing fused silica is provided which includes the steps of providing a furnace including a plurality of burners disposed above a collection surface and a refractory surface surrounding at least a portion of the collection surface. According to this aspect of the invention, the method further includes collecting soot particles on the collection surface to form a fused silica boule in a generally planar shape having an upper major surface and sidewalls. Still according to this aspect, the method further includes the step of maintaining the temperature of the refractory surface at least 300xc2x0 C. cooler than the temperature of the deposition surface.
According to another aspect of the invention, the method may further include the step of maintaining an essentially constant distance between the upper major surface of the boule and the burners during formation of the boule. In another aspect of the invention, the collection surface is generally planar and does not include lateral walls in contact with the sidewalls of the boule. According to another aspect, the furnace further includes exhaust ports positioned at the same level or below the collection surface. Still another aspect of the invention relates to maintaining the hydrogen concentration in the fused silica below 3xc3x971017 molecules/cm3. In another aspect of the invention, the deposition surface temperature is maintained to at least 1800xc2x0 C. and the crown refractory surface temperature is maintained to at least 1350xc2x0 C. during the step of collecting the soot particles on the collection surface.
Another aspect of the invention relates to an apparatus, and more particularly, a furnace for manufacturing fused silica boules. According to this aspect, the furnace includes a plurality of burners for depositing fused silica particles and a generally planar collection surface disposed below the burners for collecting the fused silica particles and supporting a boule having an upper major surface and sidewalls during the deposition process. According to this aspect, the collection surface lacks lateral walls in contact with the boule sidewalls.
In another aspect, the furnace further includes means for maintaining a constant distance between the burners and the upper major surface of the boule during formation of the boule. In still another aspect, the furnace further includes vents disposed at the same level or below the collection surface. According to another aspect of the invention, the furnace further refractories surrounding the fused silica boule, and the furnace further includes means for maintaining the temperature of the refractories at least 300xc2x0 C. cooler than the temperature of the collection surface.
The invention also relates to fused silica members produced in accordance with the method and apparatus of the invention. The methods and apparatus of the present invention are capable of producing fused silica member having an internal transmission of at least 99.75% at 193 nm. In addition, fused silica members can be produced according to the present invention having a refractive index gradient in a direction substantially perpendicular to the use axis of less than 3 parts per million up to a thickness of 100 mm. The fused silica members produced by the methods and apparatus of the present invention will enable the production of lens systems exhibiting lower absorption levels within lens systems used in photolithographic equipment. Lower absorption will reduce lens heating effects, which impacts image performance and loss of contrast in photolithographic systems.
Additional advantages of the invention will be set forth in the following detailed description. It is to be understood that both the foregoing general description and the following detailed description are exemplary and are intended to provide further explanation of the invention as claimed.